1. Field of the Invention
The present invention relates to an image display device in which a plurality of pixels are arranged in matrix and a method of testing the image display device.
2. Description of the Related Art
In recent years, image display devices such as a liquid crystal display (LCD), an electroluminescence (EL) display and the like have been advanced in precision and thus the degree of integration of elements has been remarkably improved as well.
It is an essential part of the production line of image display device to test if a circuit mounted on a substrate operates normally before shipment as a finished panel. The test process itself has been becoming more complicated in accordance with the higher precision.
FIG. 3 shows a configuration of a substrate on which a testing circuit is implemented in a conventional image display device. Such a configuration is shown, for example, in Japanese Patent Laid-Open No. 14-116423. The image display device comprises a substrate 301, a source driver circuit 302, a gate driver circuit 303, a pixel 304, a data signal line 305, a scanning line 306, a testing circuit 311, a switch driver circuit 312, an analog switch 313, a testing line 314, and testing terminals 315a and 315b. The substrate 301 mounts the testing circuit 311 and the pixel 304 which are arranged in matrix, and the data signal line (source bus line) 305 and the scanning line (gate bus line) 306 are arranged so as to be orthogonal to each other. The scanning line 306 is connected to the gate driver circuit 303 and the data signal line 305 is connected to the source driver circuit 302.
A pixel portion of the display device comprises a first pixel, which is provided in a display region and contribute to an image display and a second pixel, which is provided in outer edges of the display region (hereinafter referred to as a non-display region) and have no influence on the image display. FIG. 7A shows an overview thereof. The image display device comprises a substrate 701, a source driver circuit 702, a gate driver circuit 703, a pixel portion 704, a data signal line 707, and a scanning line 706. A non-display region 704b is provided at one side of outer edges of a display region 704a for simplicity, however, it is often the case that the non-display region 704b is provided so as to surround the outer edges of the display region 704a. 
In the display device, each pixels connected to the scanning line 306 is controlled by the scanning line 306. A video signal is sequentially supplied to the source driver circuit 302 and the video signal is outputted simultaneously to the data signal line 305 when a latch signal is inputted, and afterwards inputted to each pixel.
The configuration of a pixel in the display device is explained in details with reference to FIG. 7B. Each of the first and second pixels comprises a data signal line 711, a scanning line 712, a current supply line 717, a switching thin film transistor 713 (thin film transistor is hereinafter referred to as TFT), a driver TFT 714, a capacitor 715, a light emitting element 716, and a power source line 718.
The switching TFT 713 is provided between a gate electrode of the driver TFT 714 and the data signal line 711, and the scanning line 712 is connected to a gate electrode of the switching TFT 713. Therefore, the driver TFT 714 can be controlled by a signal outputted to the data signal line 711 when the switching TFT 713 is ON. The capacitor 715 is provided between the gate electrode of the driver TFT 714 and the current supply line 717 and holds a voltage between gate and source of the driver TFT 714.
The difference between the first and second pixels is explained hereafter. In the first pixel, a light emitting element is provided between a driver TFT and a power source, a light is emitted when a current is supplied to the light emitting element by supplying a current from the current supply line through the driver TFT. The light emitting element emits a light according to the potential of video signals inputted from a source signal line to a gate electrode of the driver TFT. In the second pixel, a light emitting element is not connected to a driver TFT, hence the second pixel is a dummy pixel which does not emit light.
One reason why such a non-display region having a dummy pixel is provided in peripheral portions of a display region is that the peripheral portion of a pixel portion tends to be inhomogeneous compared with the center part thereof in the step for forming a liquid crystal element and a light emitting element in the pixel portion in a flat panel display such as a liquid crystal display device or a light emitting device. See, for example, Japanese Patent Laid-Open No. 5-241153.
A short circuit between wirings and broken wires of the display device can be detected by a method of testing an output by bringing a probe pin into contact with the detecting pad 315a, or by a method of using a testing circuit 311. The testing circuit 311 supplies a potential level of each data signal line to a testing line 314 sequentially while driving a switch control circuit 312, then a short circuit between wirings and broken wires of the display device can be detected by bringing a probe pin into contact with the testing terminal 315b. 
The method of testing by bringing a probe pin into contact with the testing terminal 315a requires a lot of time since every testing terminal of every testing line has to be tested. Instead, when a number of probe pins is increased for saving time, expensive testing apparatus is required. As another idea, it is suggested that the testing circuit 311 is formed over a substrate, however, the testing circuit requires a large space since the switching driver circuit 312 is required in the testing circuit, although the testing circuit 311 has nothing to do with the image display.
An image display device having a testing circuit without a switching driver circuit is suggested (for example, U.S. Pat. No. 2,618,042). FIG. 4 is a configuration diagram of the image display device which comprises a substrate 401, a source driver circuit 402, a gate driver circuit 403, a pixel 404, a data signal line 405, a scanning line 406, a video signal line 407, analog switches 408 and 412, a testing circuit 411, a testing line 413, and testing terminals 414a and 414b. The data signal line 405 is connected to a gate of the analog switch 412.
Faults such as broken wires of the data signal lines 405 can be detected by inputting a testing pulse to the video signal line 407 and observing an output waveform from the analog switch 412 at a testing terminal 414b. 